There is an optical scanning apparatus used in electrophotography image forming apparatus that have the following configuration. That is to say, optical scanning apparatuses which deflect a light beam emitted from a light source by a rotational polygonal mirror, and guide the deflected light beam to a photosensitive face of a photosensitive member by optical parts such as lenses and mirrors and the like, thereby forming a latent image on the photosensitive member. FIG. 6 illustrates an overview of components of an optical scanning apparatus that is conventionally generally employed. Note that detailed description of an optical scanning apparatus will be made in the later-described embodiments. In a case where a contaminant such as dust or the like adheres to an internal optical part of an optical scanning apparatus, the quantity of light of the light beam on the photosensitive member deteriorates due to the adhered dust shielding the light ray, leading to occurrence of variance in concentration. Pollution of the atmosphere has led to an increase in minute particles and chemical substances in the atmosphere that are 1 μm in size or smaller, so deterioration in image quality due to soiling of the optical parts has become more problematic than ever before.
Various methods are being used to prevent external intrusion of dust and the like into the optical scanning apparatus, such as measures of sealing off by placing foamed members in gaps at peripheral portions of the optical scanning apparatus, measures of applying tape, and so forth, but there is need for even higher airtightness due to the following reasons. Demand for faster image forming speed in recent years has necessitated faster rotational speed of the rotational polygonal mirror as compared to conventional products. The rotational polygonal mirror disposed within the optical scanning apparatus generates airflow due to high-speed rotations. Wind flows through minutes spaces connecting the continuously connected bubbles at places where the foamed material has been adhered, and air flows from the inside out in some places while air flows from the outside in at some places. The air flowing from the outside in contains minute particles that soil the optical parts, so the more the apparatus runs, the more minute particles intrude therein, and adhere on the surface of the optical parts on the inside of the housing. Particularly, dust contained in the airflow around the rotational polygonal mirror adheres to the reflecting faces of the rotational polygonal mirror rotating at high speed. That is to say, rotation of the rotational polygonal mirror causes Karman vortex and turbulence near the reflecting faces of the rotational polygonal mirror, and the airflow bearing the dust violently collides with the reflecting faces. As a result, the minute particles colliding with the reflecting faces of the rotational polygonal mirror are accumulated, soiling progresses from portions where more airflow collides, and the reflectance of the soiled portions deteriorates. The quantity of light of the light beam guided onto the photosensitive member decreases as the reflectance decreases due to such contamination, thereby causing the problem that the output image concentration is reduced.
Optical scanning apparatuses need an opening for outputting the light beam from within the optical scanning apparatus to the outside, in order to guide the light beam deleted within the optical scanning apparatus on the photosensitive member. Accordingly, optical scanning apparatuses always have an opening for emitting the light beam to the outside of the optical scanning apparatus. A mainstream measure to avoid the soiling described above is to apply a glass member by two-sided tape. PTL 1 also proposes a configuration where part of the glass member is fixed by adhesion to the housing.